Add plug-in summary for osc doc

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http://docs.openstack.org/developer/python-openstackclient/plugin-commands.html

Change-Id: Id6339d11b900a644647c8c25bbd630ef52a60aab

barbicanclient/osc_plugin.py

@@ -10,6 +10,8 @@
 #   License for the specific language governing permissions and limitations
 #   under the License.
 
+"""OpenStackClient plugin for Key Manager service."""
+
 from barbicanclient import client